The invention generally relates to imaging systems, and particularly relates to systems and methods for producing high quality images using light modulators such as diffractive light modulators.
Systems for producing high quality images using light modulators typically include an illumination source for producing an illumination field, a light modulator for receiving the illumination field and for producing a modulated illumination field, and imaging optics for directing the modulated illumination field toward an imaging surface. For example, U.S. Pat. No. 6,433,934, the disclosure of which is hereby incorporated by reference, discloses an imaging system that includes an illumination source (e.g., an array of laser diode emitters), a field lens system, a light modulator, imaging optics and an imaging drum for supporting recording media.
Other systems, such as that shown in FIG. 1, may include an illumination source 10 having high power laser beams (800 nm-1064 nm to 40 Watts output) that are generated by one or more lasers for exposing film or infra-red (IR) sensitized media at an imaging plane 20. The system further includes a light modulator 12, a first lens 14 having a focal length of f1, a pupil 16, and a second lens 18 having a focal length of f2. The IR sensitized media is selectively exposed to create half-tone images onto imaging media (often referred to as plates) used by press systems in publishing. The media is then chemically processed and fed into a press system for printing and publishing. The high power laser beam that is generated may form a line of illumination that may be directed toward, and subsequently selectively reflected by, the spatial light modulator or grating light valve (GLV) 12. As shown in FIG. 2, a grating light valve may include ribbons 21, 22, 23, 24, 25, 26 that permit the selective imaging of each individual pixel onto the imaging media. The ribbons may each have a length L, a width w and a ribbon center to center distance a. For example, Silicon Light Machines of Sunnyvale, California sells grating light valves suitable for imaging. During operation, ribbons 21, 23 and 25 may be selectively activated to provide either a smooth reflective surface (when aligned with ribbons 22, 24 and 26) or a diffractive surface (when moved ¼ wavelength with respect to ribbons 22, 24 and 26). With reflective light modulators, the first order reflection may for example, be used for imaging, while a second order (and to a lesser extent third and fourth order etc.) reflected fields may produce unwanted illumination that is filtered out in the Fourier plane by a slit diaphragm.
Typically, a portion of a light modulator may be selectively activated to either illuminated or not illuminate an individual pixel at an imaging surface. Through this method, half tone imaging may be accomplished. Each individual pixel may be selected to be either on or off. It has been found, however, that non-linear diffraction behavior may occur in certain high quality imaging system, particularly if only a single pixel or a small group of pixels are illuminated in a relatively larger area of the imaging surface. This non-linear diffractive behavior may result in a single pixel or small group of pixels having either too strong an intensity or a poorly defined boundary for the pixel or pixels.
There is a need, therefore, for a high resolution imaging system having improved image quality.